In a power station in which two or more steam generators are connected to a steam turbine, for running turbines alongside one another or connection of a second or third steam generator the steam systems must be brought to approximately the same the steam states (pressure, temperature) to enable the systems to be connected by opening the shut-off valves.
A specific time is needed for this and mostly the steam generator power must also be kept constant in such cases so that the most stationary and stable possible states are present during the coupling. If this is not the case variations in pressure, temperature and/or quantity of steam can result in problems within the steam generator systems (drum level changes) or the operation of the steam turbine (limit values being reached, steam turbine trip). This means that a specific time is needed for this synchronization and the power plant output cannot be increased as quickly as desired. In addition the steam generated during the connection process is discarded via the bypass station into the condenser and the efficiency of the plant reduced thereby.
A method is known in which the steam systems are brought up to approximately the same pressure and temperature in accordance with a specific logical sequence and the system is coupled in a specific sequence. For example with three-pressure systems with simple intermediate superheating, first of all the cold intermediate superheating system is set to the same steam parameters and coupled by opening the cold intermediate superheating slider to the intermediate superheating system in operation with the steam turbine. Then the high-pressure steam system of the second steam generator is connected in the same way with the first steam generator in operation. Subsequently, when the pressure and temperature are equal, the hot intermediate superheating system is also coupled by the steam slider in the hot intermediate superheating system being opened. The bypass stations in such cases are each closed in a controlled manner and thereby a greater load is imposed on the steam turbine. In the same way after these steps the low-pressure steam system is also connected to the second steam generator, the disadvantage of this method is that it is very time-consuming.
As an alternative to this, after the harmonization of the different pressure and temperature levels, the shut-off valves of the different pressure systems can be opened at more or less the same time and the second steam generator is thus coupled to the first steam generator already in operation with the steam turbine. In this case to the steam generator power (thus also the power of the gas turbine for combined cycle plants) must be kept constant, i.e. a further starting up of the steam generator is stopped. This method, although faster, conceals the additional risk however of problems developing in a number of systems of the same time and possibly influencing each other. This method makes an unwanted shutdown of the steam turbine or of the steam generator significantly more likely because limit values have been exceeded (drum level high or low, too little or too much steam in the steam turbine direction).